(a) Field of the Invention
The present invention relates to a fan speed control circuit and, more particularly, to a fan speed control circuit capable of reducing vibrations and noises in a fan motor by using a phase control method.
(b) Description of the Related Art
Among conventional fan speed control technologies, pulse width modulation (PWM) control is the most common method used to control the speed of direct current (DC) motors.
Referring to FIG. 1, in a conventional PWM control method, an external PWM signal 102 is used to drive a bipolar transistor 104 and a driver IC 106 such that the terminal voltage of a stator coil 108 is controlled. Also, a hall element 110 whose output voltage is directly proportional to the magnetic field strength is used to sense the magnetic polarity of the rotor in a fan motor.
Referring to FIG. 2, the fan motor is “on-time” as the pulse level of the incoming PWM signal 102 is high (also called the duty cycle), while the fan motor is “off-time” as the pulse level is low. The sum of these two times is one period. Therefore, one can, for instance, decrease the on-time or increase the off-time of the fan motor in one period to lower the motor speed.
In a brushless DC motor, cogging torque is produced by the magnetic attraction between the rotor mounted on permanent magnets and the stator, and it is an undesired effect that makes precise positioning of the rotor impossible because the rotor tends to lock onto the position where it is aligned with the stator poles. Further, when the rotor rotates, the magnetic flux variation causes back electromotive force (back emf) effect, and the back emf effect in turn enhances the cogging torque to result in high values of output ripple, vibrations, and noises in the fan motor.